Process for making a pellet

ABSTRACT

The invention comprises a shaping process for making pellets of a thermoplastic extrudable resin composition. The resin composition comprises a thermoplastic polymer, plasticiser and optionally further additives. The plasticiser comprises a component which is solid at room temperature. The process is run at a temperature above the melting point of the plasticiser and below the melting/plastification temperature of the thermoplastic polymer.

The present invention relates to a process for making pellets of athermoplastic extrudable polymer.

Processes for making pellets of thermoplastic extrudable polymer arewell known in the plastic industry. Typically the pellets arecylindrical and approximately 3 mm in diameter and 3 mm in length. Thepellets are used in a wide range of plastic article manufacturingprocesses.

The pellet manufacturing process generally includes a plastificationstep. In this step the formulation to be pelletised is melted and fedinto a twin screw extruder. This has been seen to be beneficial as thepellets produced have been found to comprise of a homogeneous blend ofthe pellet components due to effective mixing of all molten componentsin the extruder.

EP-A-0 415 357 describes the making of pellets comprisingpolyvinylalcohol (PVOH) by melt extrusion with the extrusion beingcarried out in the temperature range of 150-195° C.

Pelletising processes having a plastification step have severaldisadvantages associated therewith. The principle disadvantage is therequirement for heating, which means that the energy consumption ofthese processes is very high.

Furthermore these ‘hot’ processes are not suitable for polymers whichare heat sensitive (such as PVOH) due to heat induced decomposition.Also these ‘hot’ processes give a heat history to the polymer which hasbeen found to negatively influence properties of the polymer. In thecase of PVOH this has been found to detrimentally affect the PVOH watersolubility.

In other pelletising processes dry compaction of the pellet componentsis carried out at low temperature. Thus the disadvantages of the ‘hot’processes are avoided.

WO-A-98/26911 describes a low temperature process for the manufacture ofPVOH pellets. In the process the pellets components, in this case amixture of powdered PVOH and various additives such as plasticisers isfed between two rollers and compressed into pellets. The PVOH componentin the pellet blend is not melted in the process and so the issue ofheat degradation is avoided.

Also GB-937 057 describes such a low temperature compression process.This follows initial mixing of the plasticiser and PVOH at an elevatedtemperature.

However, although this process (the cold compression process) eliminatesthe problem of heat induced decomposition of the polymer, the pelletsproduced suffer from other disadvantages.

Most of the disadvantages stem from the inherent nature of thecompaction process, more specifically the rollers and the powder feedthereto. It has been found to be very difficult to ensure that thepowder feed is spread evenly across the rollers. This has the effectthat control of the size of the pellets is difficult and so the size ofthe pellets can vary significantly.

Furthermore significant dust formation is typical for this kind ofprocess. Additionally the pellets are commonly friable having poorintegrity and easily form dust from friction rubbing against each other,thus worsening the dust issue. Both of these issues are attributed tothe poor spreading and roller compression technique.

Furthermore significant variability of the composition of the pelletsand poor homogeneity of the pellets has also been observed. The issuesare also believed to be associated with the poor powder distributionover the rollers. The problem of the variability of the pelletcomposition and the poor homogeneity of the pellets is exacerbated whenthe pellets are taken and used in a further processing step.

These kinds of pellets, wherein the thermoplastic polymer component ofthe pellets is PVOH, are used in the manufacture of water soluble PVOHpouches in extrusion/injection moulding processes. The pouches, as anexample, are commonly used to contain a detergent composition for use inan automatic washing machine (laundry/dishwasher). In these applicationsis it vital that the pellets have high homogeneity to ensure that thepouches produced have good integrity to be stable in storage and havethe expected water dissolution properties.

Pellets produced in a cold compaction process, as described above, oftenfail to meet the level of homogeneity required for the processing intothe pouch format.

It is an object of the present invention to obviate/mitigate theproblems outlined above.

According to the present invention there is provided a shaping processfor making pellets of a thermoplastic extrudable resin compositioncomprising a thermoplastic polymer, plasticiser and optionally furtheradditives, the plasticiser comprising a component which is solid at roomtemperature, wherein the process is run at a temperature above themelting point of the plasticiser and below the melting/plastificationtemperature of the thermoplastic polymer.

The shaping process may comprise pressing, extrusion, calendering and/orcompaction. Most preferably the shaping process comprises extrusion.

The process of the present invention has been found to overcome thedisadvantages associated with the prior art. Firstly as the process isoperated at a temperature below the melting/plastification temperatureof the thermoplastic polymer the process has been found to be extremelyenergy efficient. Furthermore the heat degradation of heat sensitivematerials in the resin blend is dramatically reduced by the loweredprocess temperatures.

Additionally as the process operates above the melting point of theplasticiser (which is then allowed to cool to form the solid pellet) thepellets have been found to have a very low friability. Thus the pelletshave a much lower tendency to release dust upon friction rubbing.

Furthermore as the pellets are produced at a temperature above themelting point of the plasticiser component the pellets have been foundto have excellent homogeneity. More specifically both the overallcomposition of each pellet and the distribution of the individualcomponents within the pellets have been found to have an high level ofpredictability and low variance. This is especially important when thepellets are used in a further processing step such as a second extrusionprocess (e.g. injection moulding) for the manufacture of an articlecomprising the thermoplastic polymer.

Generally the components are delivered to the shaping equipment used inthe process in particulate form.

It has been found that he particle size of the raw materials used tomake the pellets should be small. This has been observed to ensure highhomogeneity of the pellets. The particle size of the raw materials usedpreferably is below 2000 μm, more preferably below 1200 μm, morepreferably below 400 μm and most preferably about 200 μm.

Preferably the plasticiser is present in the composition with at least5%, more preferably 10%, most preferably 15%.

Preferably the temperature of the material within the extruder does notexceed a temperature which is 10° C. below the melting/plastificationtemperature of the thermoplastic polymer at any time. More preferably itdoes not exceed 15° C., more preferably 30° C. and most preferably 45°C. below the melting/plastification temperature of the thermoplasticpolymer. However, it is desired that the temperature of the materialexceeds the ambient air temperature. Preferably the temperature ofmaterial within the extruder is at least 40° C., more preferably atleast 45° C., and most preferably at least 50° C.

The plasticiser has to at least partially melt at the preferredoperating temperature. The melting point of the plasticiser component ispreferably at least 15° C., preferably at least 30° C. and mostpreferably at least 45° C. below the melting/plastification temperatureof the thermoplastic polymer.

Preferably the plasticiser comprises a carbohydrate.

Carbohydrates are usually represented by the generalised formulaC_(x)(H₂O)_(y). The term herein also includes materials which aresimilar in nature like gluconic acids or amino sugars which cannot befully represented by this formula. Other carbohydrate derivatives likesugar alcohols such as sorbitol, glucitol, mannitol, galactitol,dulcitol, xylitol, erythritol, isomaltutose and isomalt fall within thisterm.

Most preferred carbohydrates include the more thermally stablecarbohydrates such as sorbitol, glucitol, mannitol, galactitol,dulcitol, xylitol, erythritol, isomaltutose and isomalt.

Other preferred plasticiser systems include solid fatty acidalkoxylates, fatty alcohol alkoxylates or polyalkylene glycols (such aslong chain polyethylene glycol).

The plasticiser may comprise a further auxilliary component. Preferredauxilliary components include glycerin, ethylene glycol, propyleneglycol, diethylene glycol, diproylene glycol, triethanol amine,diethanol amine and methyldiethyl amine.

Once the or each strand has issued from the extruder it may be permittedto cool under ambient conditions. Alternatively cooling may be assisted.One way in which this may be done is by employing a cooled metal beltonto which the or each strand issues. Another way in which this may bedone is by using a cooled fluid, preferably cooled air, downstream ofthe extruder. Another way is by blowing a fluid, preferably air, acrossthe or each strand. One or more of these methods may be used.

Preferably the or each strand is separated into pellets, during themanufacture.

The strands are separated into pellets preferably by cutting. However,other separation methods, for example twisting, are not ruled out. Amethod may be envisaged whereby the strand is twisted at intervals whenstill plastic, to form “sausages”, which can be separated by breakingthe connections, once they have become more brittle. Partial cutting orpressing or nipping or perforating (all such methods collectively called“scoring” herein) to form frangible separation webs, may also beemployed, to form tablet precursors. Separation of the precursors toproduce pellets may be effected during manufacture or by the consumer,manageable lengths being provided from which the consumer breaks ortwists off pellets as required. A pellet precursor may be, for example,a straight row of pellets, to be broken off as needed.

The extrusion pressure may be whatever is required to carry out theprocess in an efficient manner. Suitably it is in excess of 3 bar (0.3MPa), preferably in excess of 5 bar (0.5 MPa), and more preferably ispreferably in excess of 8 bar (0.8 MPa). More preferably still ispreferably in excess of 12 bar (1.2 MPa). Most preferably it is inexcess of 40 bar (4 MPa). The extrusion pressure preferably does notexceed 100 bar (10 MPa), more preferably 60 bar (6 MPa).

Generally the pellets are for use in injection moulding processes. Theinjection moulding process is preferably used for the manufacture ofwater soluble pouches intended to contain a detergent formulation foruse in an automatic washing machine or in an automatic dishwasher. Thusthe pellets preferably comprising a water-soluble/water-dispersiblethermoplastic polymer

In this use the advantageous properties of the pellets produced inaccordance with the invention, especially the high homogeneity have beenfound to be particularly beneficial. It is believed that this propertyis most beneficial as the integrity of the injection moulded productrelies upon such high homogeneity of the composition being injectionmoulded as otherwise the low homogeneity will be reflected in theinjection moulded product. The high homogeneity has been found to leadto predictable water solubility of injection moulded products.

Preferably the water-soluble/water-dispersible thermoplastic polymercomprises PVOH or a derivative thereof.

Other water-soluble/water-dispersible polymers may be used in theprocess either as an alternative or in addition to PVOH. Preferredexamples include poly(vinylpyrollidone), poly(acrylic acid), poly(maleicacid), a cellulose derivative (such as a cellulose ether/hydroxypropylmethyl cellulose), poly(glycolide), poly(glycolic acid), poly(lactides),poly (lactic acid) and copolymers thereof.

Processing aids may be present in the admixture which is processed.Preferred processing aids include mono-, di-, tri-carboxylic acids/saltsthereof, fatty acids such as stearic acid/salts thereof, mono-, di- ortriglycerides/salts thereof, aerosil, inorganic and organic pigments.

The invention will now be illustrated with reference to the followingnon-limiting Examples.

EXAMPLES Example 1

The pelletising process was conducted on an extruder (twin screw, ICMAS. Giorgio, Milan (dedicated to processing of plastic blends andalloys).

The extruder had the following characteristics. Screw diameter: 35 mmScrew length: 40 cm Working pressure: 30 bar Output:  5 kg/h.Temperature zones: 6 (T1 = 50° C., T2 = 60° C., T3 = T4 = 90° C., T5 =105° C. and T6 (the die) = 105° C.)

The extruder was attached to a two-roll unit used as a cooling sourceand connected to a pellet cutter.

The following formula was fed into the extruder in powder form. Material% PVOH resin 85.0 Sorbitol 11.0 Processing aids 4.0 Total 100.0

The pellets obtained were chilled to room temperature. The formulayielded solid pellets having low friability.

Example 2

The pelletising process was conducted on a pellet press (model V3-75from Universal Milling Technologies).

The press had the following characteristics. Die diameter: 350 mm Holesdiameter:  2 mm Hole length:  3 mm Infeed cone: 45° Space betweendie/rollers:  1.5 mm Die speed: 5 m/s Motor: 30 kW Temperature: 98-102°C.

The following formulae were fed into the extruder in powder form.Material Formula 1 Formula 2 Formula 3 PVOH resin 81.0 87.0 85.0Sorbitol 15.0 11.0 11.0 Processing aids 4.0 2.0 4.0 Total 100.0 100.0100.0

The pellets obtained were chilled to room temperature. Each formulayielded solid pellets having low friability.

1. A shaping process for making pellets of a thermoplastic extrudableresin composition comprising a thermoplastic polymer, plasticiser andoptionally further additives, the plasticiser comprising a componentwhich is solid at room temperature, wherein the process is run at atemperature above the melting point of the plasticiser and below themelting or plastification temperature of the thermoplastic polymer.
 2. Aprocess according to claim 1, wherein the process comprises pressing,extrusion, calendaring or compaction.
 3. A process according to claim 1,wherein the plasticiser is present in the composition in at least
 5. 4.A process according to claim 2, wherein the shaping process comprisesextrusion.
 5. A process according to claim 4, wherein the temperature ofthe material within the extruder does not exceed a temperature which is110° C., below the melting or plastification temperature of thethermoplastic polymer at any time.
 6. A process according to claim 4,wherein the temperature of material within the extruder is at least 40°C.
 7. A process according to claim 1, wherein the particle size of theraw materials used is below 2000 μm.
 8. A process according to claim 1,wherein the plasticiser comprises a carbohydrate.
 9. A process accordingto claim 8, wherein the carbohydrate is selected from gluconic acids,amino sugars, sugar alcohols such as sorbitol, glucitol, mannitol,galactitol, dulcitol, xylitol, erythritol, isomaltutose and isomalt. 10.A process according to claim 8, wherein the carbohydrate is selectedfrom sorbitol, glucitol, mannitol, galactitol, dulcitol, xylitol,erythritol, isomaltutose and isomalt.
 11. A process according to claim1, wherein the thermoplastic polymer is water-soluble or waterdispersible.
 12. A process according to claim 11, wherein thethermoplastic polymer comprises PVOH or a derivative thereof.
 13. Aprocess according to claim 1, wherein the thermoplastic polymercomprises poly(vinylpyrollidone), poly(acrylic acid), poly(maleic acid),a cellulose derivative, poly(glycolide), poly(glycolic acid),poly(lactides), poly (lactic acid) and copolymers thereof.
 14. A processaccording to claim 12, wherein the pellets are for use in injectionmoulding processes.
 15. An injection molding process for manufacturingwater soluble pouches containing a detergent formulation for use in anautomatic washing machine or an automatic dishwasher according to claim12.
 16. A process according to claim 3, wherein the plasticiser ispresent in the composition in at least 10%.
 17. A process according toclaim 16, wherein the plasticiser is present in the composition in atleast 15%.
 18. A process according to claim 5, wherein the temperatureof the material within the extruder does not exceed a temperature whichis 15° C., below the melting or plastification temperature of thethermoplastic polymer at any time.
 19. A process according to claim 18,wherein the temperature of the material within the extruder does notexceed a temperature which is 30° C., below the melting orplastification temperature of the thermoplastic polymer at any time. 20.A process according to claim 18, wherein the temperature of the materialwithin the extruder does not exceed a temperature which is 45° C., belowthe melting or plastification temperature of the thermoplastic polymerat any time.